A flexural plate wave (FPW) sensor includes a diaphragm or plate which is driven so it oscillates at frequencies determined by a comb pattern and the flexural plate geometry. The comb pattern is disposed over the flexural plate and establishes electric fields which interact with the plate's piezoelectric properties to excite motion. The eigenmodes describe the diaphragm displacements which exhibit spatially distributed peaks. Each eigenmode consists of n half sine periods along the diaphragm's length. A typical FPW sensor can be excited to eighty or more eigenmodes. In a typical FPW eigenmode, the plate deflection consists of many sinusoidal (or nearly sinusoidal) peaks.
Prior art flexure plate wave sensors typically include drive combs at one end of the plate and sense combs at the other end. The drive combs of these prior art devices typically cover only twenty-five to forty percent of the total length of the plate. When the number of drive teeth is small compared to the number of eigenmodes peaks, the small number of drive teeth can align with several eigenmodes. The result is that not only are the eigenmodes perfectly aligned with the comb teeth excited, but other eigenmodes are also excited. In signal processing and spectral analysis, this effect is known as leakage. A significant drawback of prior designs is that the increased number of eigenmodes excited in the FPW sensor produces a series of resonance peaks of similar amplitude and irregular phase which increases design complexity and the operation of the prior art flexure plate wave sensors.
Moreover, prior art flexural plate wave sensors utilize drive and sense combs at opposite ends of the flexural plate and rely on analysis based on an analogy to surface acoustic waves (SAW) wherein the waves propagate away from the drive combs and toward the sense combs and back reflections are regarded as interference. A distinct disadvantage of this analysis is that SAW theory does not account for numerous small peaks produced by the sensor resulting in calculated gains (e.g., peaks of similar magnitude) which are low and do not account for sharp phase drops seen with the peaks (e.g., irregular phase).